Determining who has cited a work
There are usually multiple ways to find out who has cited an article or other scholarly work. One way to is to use the Cited By tool in Google Scholar. The other is to use Cited By tools in library databases. Neither source will necessarily include perfect citation counts, as neither tracks all scholarly works that are published and there may be a lag time before new publications are included.
Library Search
Library Search will provide citation information for some articles, when available. There are two ways to find this information.
First, when looking at your list of search results, see if a record for an item has a red double-headed arrow pointing up that reads "Find sources citing this" when your mouse is over it. You can select that symbol to see a list of all the items that cite the original article. To see the total number, look at the number of results.
Second, you can click into a record for an item and go to the section labeled "Citations" to see if it provides the number of citations from Web of Science. You must be logged into your Library Search account to use this feature.

Library databases
A number of library databases also offer a similar service by tracking which other scholarly works have cited an item. For instance, in Web of Science, you can see how many times an article has been cited by looking at the right side of each entry. (Web of Science data is also displayed in Library Search article results.) Other databases offer a similar service—just look for mentions of “cited by” or other similar phrases when looking at an article record to see if that database offers the service.

Most databases do not track all scholarly works, meaning they will likely miss some works that have cited a particular article or book. Thus, these citation counts are likely incomplete.
Google Scholar
After conducting a search in Google Scholar, it will include the number of times each article in the results list has been cited by other people. You can click on this to go explore those works.

Be aware that the Cited By tool can feature errors. Google Scholar uses automated algorithms that may create double counting of the same citation or include false positives.